1. Field of the Invention
The present invention relates to a flat panel display device and a method for fabricating the same.
2. Discussion of the Related Art
In recent years, flat panel displays (FPD) are increasing in importance as multimedia is developed. In response to this, several flat type displays such as liquid crystal display (LCD), plasma display panel (PDP), field emission display (FED), and organic light emitting device (OLED) are being put to practical use. Among them, the liquid crystal display has an excellent visibility, and has lower average consumption power and heat release rate compared with a cathode ray tube. The field emission display is attracting attention as a next generation flat panel display device because it has a great response speed of 1 ms or less, has low power consumption, and has no viewing angle problems because of its self-emission.
Methods for driving flat panel display devices are classified into a passive matrix method, and an active matrix method using a thin film transistor (TFT). The passive matrix method is a method in which an anode and a cathode are formed to cross at right angles, and a line is selected, thereby driving the flat panel display device. In comparison with this, the active matrix method is a method in which the thin film transistor connects to each pixel electrode of indium tin oxide (ITO), and the flat panel display device is driven depending on a voltage sustained by the capacitance of a capacitor connecting to a gate electrode of the thin film transistor.
FIG. 1 is a cross-sectional view illustrating a conventional flat panel display device. Referring to FIG. 1, the flat panel display device comprises a first substrate 100, a light emitting unit 110, and a second substrate 120 opposing the first substrate 100. The first substrate 100 and the second substrate 120 are sealed by a sealant 130, thereby sealing the light emitting unit 110. The light emitting unit 110 may comprise a first electrode, a second electrode, and an emission layer or a liquid crystal layer interposed between the first and second electrodes. The flat panel display device based on the active matrix method may further comprise a thin film transistor electrically connecting with the first electrode.
A substrate used for the flat panel display device may be made of glass, plastic, or metal. In general, the substrate formed of glass is typically used. The glass may be a non-alkali glass, a soda lime glass, or a borosilicate glass. The non-alkali glass contains less than 0.1 wt % of Na2O, the borosilicate glass contains 0.1 wt % to 1 wt % of Na2O, and the soda lime glass contains more than 1 wt % of Na2O. The soda lime glass is also called an alkali glass.
In the flat panel display device comprising the thin film transistor based on the active matrix method, non-alkali glass is typically employed as the first substrate. This is to protect the thin film transistor from alkali ions diffused from the substrate in a process of fabricating the thin film transistor. In other words, when the alkali ions are diffused into a channel region of a semiconductor layer, the alkali ions change a semiconductive property of the channel region into a conductive property. This deteriorates an off characteristic of the thin film transistor, increases a leakage current, and causes a residual image while driving the display. Accordingly, it is desirable to employ non-alkali glass in the first substrate in a flat panel display device using the active matrix method, in order to solve the above problems. However, the non-alkali glass substrate is more expensive than other glass substrates and thus, may increase a price of the flat panel display device. In order to reduce a manufacture cost, the first substrate having the thin film transistor may employ the non-alkali glass, and the second substrate sealed with the first substrate may employ the soda lime glass.
The first substrate comprising the thin film transistor and the light emitting unit is sealed with the second substrate by the sealant, thereby sealing the light emitting unit formed on the first substrate. In order to cure the sealant, the sealant is subject to irradiation of ultraviolet rays and then, is heat-treated for about one hour at a temperature of about 230° C.
The heat treatment causes a thermal expansion of the sealed first substrate and second substrate. Because the first substrate and the second substrate are of materials different from each other, there is a drawback in that the sealed first substrate and second substrate are bent in one direction as shown in FIG. 1. In other words, a difference between thermal expansion coefficients of two substrates may cause the bending of the flat panel display device. This causes a deterioration of yield and a reduction of reliability.